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The National Renewable Energy Laboratory (NREL) has been monitoring the performance of a 1- kWAC United Solar Systems Corporation (USSC) roofing system over the 6-year period from October 1998 to September 2004. This paper investigates the performance and reliability of this system. The annual degradation and seasonal fluctuation of the system's power output are calculated using the PVUSA power rating regression model. The system performance is also examined using the additional performance parameters of yield, reference yield, and performance ratio, which allow for a somewhat less complicated data collection and analysis. The data indicate that the system has exhibited stable performance over time, with an overall degradation rate comparable to that found in crystalline silicon systems.